Emulsion polymerisation is widely used for waterborne polymer latex coatings1. Reducing the latex particle size in such coatings is known to promote coalescence and hence enhance film formation. However, achieving smaller latex particle sizes by emulsion polymerisation usually requires additional surfactant, which can compromise the quality of waterborne coatings. For example, poor adhesion and reduced film quality can result because of migration of excess surfactant2. In principle, reactive surfactants offer a potentially decisive advantage over conventional surfactants in emulsion polymerisation because the former become irreversibly bound to the latex and hence cannot migrate during film formation; this allows defect-free coatings to be produced with reduced moisture sensitivity3.
Over the last two decades, controlled/living radical polymerisation techniques such as nitroxide-mediated polymerisation4, atom transfer radical polymerisation (ATRP)5 and reversible addition-fragmentation chain transfer (RAFT) polymerisation6-8 have become powerful tools for synthetic polymer chemists. There are many examples of latex syntheses based on these approaches9. For example, nitroxide-mediated living radical polymerisation has been used by Charleux10-15, El-Aasser16, Okubo17 and Georges18 to mediate the mini-emulsion polymerisation of n-butyl acrylate and styrene. ATRP has been optimised by Matyjaszewski19-22 and Okubo23-25 for the mini-emulsion polymerisation of (meth)acrylic and styrene monomers. RAFT polymerisation has been extensively exploited in the context of both emulsion and mini-emulsion polymerisation by Hawkett26-29, Charleux30-33, El-Aasser34, Cunningham35 and Zhu36. There are also a number of RAFT syntheses conducted under non-aqueous dispersion polymerisation conditions37-39.
However, the inventors believe that there are only two literature examples of the application of controlled/living radical polymerisation techniques for latex syntheses by aqueous dispersion polymerisation40,41. In each case, a relatively expensive speciality monomer was utilised for the latex core, namely N-isopropylacrylamide40 and N,N′-diethyl acrylamide41. This lack of research is perhaps surprising, because aqueous dispersion polymerisation formulations are conceptually much more straightforward than aqueous emulsion polymerisation since the initial reaction solution is homogeneous in the former case.
Recently, the inventors have reported the use of conventional (non-living) free radical chemistry for the aqueous dispersion polymerisation of a commodity methacrylic monomer, 2-hydroxypropyl methacrylate (HPMA)42. The resulting PHPMA latexes were stabilised by poly(N-vinylpyrrolidone) and the mean particle diameter could be varied from approximately 100 to 1000 nm diameter, with good control over the particle size distribution in most cases.